Fine grinding process for calcium minerals



United States Patent FINE GRINDING PROCESS FOR CALCIUlVI MINERALS Merrill E. Jordan, Walpole, and William G. Burbine, Whitman, Mass.

No Drawing. Application April 18, 1955, Serial No. 502,196

4 Claims. (Cl. 241-22) This invention relates to an improved process for producing finely powdered grades of various calcium-containting minerals and is especially effective and useful in comminution processes in which such minerals are reduced to products having an average particle size of less than about microns.

Although specialized techniques such as wet grinding, jet grinding, etc., have traditionally been considered the most suitable methods for effecting ultra fine subdivisions of solid particles, it is now well recognized that, with the closed circuit system of operation, ordinary dry grinding in tumbling type mills can be used successfully in this field. In fact, dry grinding in bell mills, rod mills and other tumbling type mills with continuous feed and closed circuit recycle of oversized material, in theory at least (assuming trouble-free operation), is basically more economical than either wet grinding or jet grinding in those cases where the finished product is to be a finely ground dry powder. Thus, with wet grinding extra costs are required for water removal, while, in jet grinding,

energy requirements are very high. Unfortunately,how-

ever, the majority of solid materials, in undergoing extra fine subdivision in the dry state, cause severe operating complications in tumbling type grindingequipment, rendering the entire process so .inefiicient that any expected operating economies are completely dissipated. Coking or clogging tendencies in. the mill are among the chief of such complications, and are exhibited in such wellknown phenomena as coating of the balls, pebbles, rollers, mill walls and other grinding surfaces, or independent agglomerate formation, etc., resulting in cushioning efiects and inetficient grinding. ese complications are not mere moisture effects but appear to be inherent in the dry grinding of many fine materials includingmost calcium-containing minerals.

It is an object of this invention to minimize the caking and clogging tendencies of calcium-containing minerals when undergoing dry grinding to a very fine state of subdivision. A very important object of this invention is to avoid difficulties suchas caking, coating or mill choking and cushioning which tend to interfere with the uniform continuous operation of -tumbling type mills when used in the dry grinding of calcium-containing minerals.

It is also an object of this invention to increase the grinding efficiency and/ or speed of ball, pebble, tube and other tumbling type mills when used to grind calciumcontaining minerals in the dry state whether with batch or continuous feed and, if the latter, whether with open or closed circuit grinding.

Another object is to provide methods for faster production of finely ground calcium-containing minerals using given equipment. Another important object is to make possible the realization in practice of the operating economies which are theoretically possible in closed circuit dry grinding in tumbling type equipment, by eliminating the hangups and generally poor handling qualities normally exhibited by finely-divided calcium-containing minerals in the dry state.

Still another object is to improve the anti-caking and general handling properties of the finished finely ground dry products.

Further objects and advantages will be made apparent by the detailed description and discussion of the invention which follows.

We have discovered that the addition of from about 0.05% to about 5.0% by weight of oxalic acid to the calcium-containing mineral greatly reduces the formation of agglomerates and other caking and clogging difiiculties in the tumbling type mill and causes a significant increase in the rate of particle size reduction and/or in the yield of product of a given fineness within a given cycle of operation under set conditions. The oxalic acid can be added to the mineral ore at any time up to and including the time at which it is fed or changed to the tumbling type of mill in which the final size reduction is effected. However, it is preferably added as a dry pow der to the coarse ground ore, e. g., after it has been crushed and screened but near the beginning of the fine grinding operation. Ideally, the ground ore which is fed to the tumbling type of equipment for final fine grinding is substantially all below about 20 mesh (Tyler) and the oxalic acid is added in finely powdered form right before, simultaneously, as, or just after the ore is fed or charged to the said tumbling type equipment.

It has been found that ammonium oxalate can be substituted for oxalic acid using equivalent amounts by weight on the basis of actual C2O4=content. However, the salt is not ordinarily preferred, since it is usually slightly more expensive and less efficient per pound than the acid.

Oxalic acid has been found to be very effective in preventing clogging, caking and other similar grinding interferences, hang-ups or complications at concentrations of only 0.05% by weight based on the weight of the ore. The optimum amount of oxalic acid for use in any given case will obviously depend on several factors, such as the nature of the ore, the fineness to which it is to be ground, the particular type and size of tumbling type grinding equipment and the method of operation, i. e. batch or continuous feed, open or closed circuit, etc. However, in general, it can be said that the preferred range of concentrations of the oxalic acid is from about 0.1 to 1.0% by weight and that seldom, if ever, will any additional benefit be gained by using more than about 2 parts oxalic acid per parts of ore. Neither will additional amounts of oxalic acid be undesirable except as an added expense which may be unnecessary unless, of course, its presence is desired in the finished product for some reason. In this connection, should more oxalic acid ever be added in the grinding process than is 'desired in the finished product, it is very easily removed by moderate heating since it sublimes at about C.

The minerals with which oxalic acid has been found so efiective as a grinding aid and anti-agglomerant are .the common calcium minerals, i. e., deposits in which calcium is the predominant cat-ion such as. limestone, calcite, chalk, whiting and other forms of pure to somewhat impure CaCOs, wollastonite, gypsum, and apatite, phosphate rock and other forms of crude: lime phosphate.

In accordance with this invention, the addition of oxalic acid to any of the above minerals, as specified, significantly increases the efiiciency with which they can be finely ground in any equipment involving a tumbling type grinding mill. Such tumbling type grinding mills include ball, pebble and rod mills of all kinds whether batch, conical, tube, preliminator, compartment, etc., regardless of the nature of the grinding media or surfaces or the method of operation, i. e, the type of tumbling action, method of discharge or the type of auxiliary classification and recycle, if any, etc. The improved efliciency or ease of grinding is especially noticeable in such processes when the finished product has an average particle size in the range below about 10 microns, e. g., 1 to 10 microns.

The operation in all these tumbling type grinding processes is alike in that the material being ground is tumbled around and about in such a manner that it interrningles with the relatively much larger tumbling bodies (balls, pebbles, rods, etc, usually between about /2 and in diameter) which are hard surfaced and virtually unbreakable under the conditions of operation. The com minution of the ore occurs as particles of it are either crushed by impact of the falling hard surfaced bodies or nipped and passed under or between these hard surfaced bodies rolling over one another.

Specific examples of the practice of this invention are described below but it should be understood that these! examples are merely illustrative and are not intended as limitations on the scope of the present invention.

Example 1 Into a 3 ft. diameter by 3 ft. long drum containing 528 pounds of porcelain balls about 1 in diameter and 162 pounds of 1 to 2 flint pebbles, there was charged 200 lbs. of wollastonite which had been coarse crushed and magnetically beneficiated and screened to between 25 mesh and +60 mesh. After milling for 5 /2 hours at 36 R. P. M. about 86% of the wollastonite charged had been reduced to below 325 mesh material.

The same ball mill was cleaned and recharged with 200 lbs. of the same -25 to +60 wollastonite. This time 1 lb. of oxalic acid was added and after 5 /2 hours milling at 36 R. P. M. it was found that 96% of the wollastonite charged had been reduced to below 325 mesh material, representing over a increase in. production of finely ground material. Moreover, the material ground in the presence of oxalic acid was easier to handle, sift, eltc.

Example 2 Using a minus 20 mesh feed of wollastonite in a 1 gallon size laboratory ball mill containing 7 lbs. of flint pebbles about in diameter, 18 hours milling reduced the average particle size of the material from 34 microns to 4.6 microns (as measured by the Fisher subsieve sizer at 0.6 porosity) when 0.05% by weight oxalic acid was added to the minus 20 mesh wollastonite. Wollastonite containing no oxalic acid was reduced from 34 microns to only about 22 microns in average particle size (Fisher) after 18 hours milling under otherwise identical conditions. It was noted that the pebbles remained clean in the run with oxalic acid present whereas they became badly coated in the run in which no oxalic was used.

Example 3 A finely ground whiting (CaCOa) was used as the charge to the 1 gallon size laboratory ball mill containing 7 lbs. of flint pebbles of about diameter. After 21 hours milling, the average particle size as measured on the Fisher subsieve at 0.6 porosity was reduced from about 7 microns to about 2.0 microns when 0.5% by weight oxalic acid had been charged with the whiting whereas it was reduced from 7 microns to 3 microns in the absence of any oxalic. Moreover, the material caked in the mill in the latter case so that the mill was hard to clean and the finished product was composed in large part of large lumps and was difficult to disperse to the ultimate particle size when used as an extender pigment or filler in liquid systems. The material ground with oxalic acid, on the other hand, was free flowing and readily dispersible in either aqueous or oleaginous vehicles.

Example 4 In a full scale plant test using closed circuit recycle of material above 325 mesh, the addition of 1.0% by weight oxalic acid to the 20 mesh wollastonite originally fed to the tumbling type grinding equipment greatly reduced the proportion of material recycled from the air separator and raised the over-all production rate of given equipment operating on a given cycle from about 1.15 tons/hr. to 1.5 tons/hr.an increase of about 30%.

Having described our invention and preferred embodiments thereof, what we claim and desire to secure by U. S. Letters Patent is:

1. A process for grinding calcium minerals comprising adding to a coarse crushed mineral in which calcium is the predominant cation a substance selected from the group consisting of oxalic acid and ammonium oxalate in an amount equivalent to at least about 0.05 part by weight of oxalic acid per parts of said mineral and subjecting said mineral while admixed with said substance to the fine grinding action of a mass of tumbling and rolling hard surfaced bodies which are large in size relative to the size of the individual particles of coarse crushed mineral.

2. A fine grinding process comprising adding about 0.05 to 5 parts by weight oxalic acid to 100 parts of a coarse crushed calcium containing mineral and tumbling the said mineral about together with said oxalic acid in the presence of a mass of rolling and tumbling hard surfaced bodies having an average equivalent spherical diameter of at least about /2 inch until the average particle size of the said mineral has been reduced to below about 10 microns.

3. The process of claim 2 in which the coarse crushed calcium mineral is wollastonite ore which has previously been crushed, magnetically beneficiated, and screened through 20 mesh screens, the added oxalic acid amounts to 0.1 to 1.0 part per 100 parts of said wollastonite by weight and the hard surfaced bodies are porcelain balls which are tumbled about together with the treated wollastonite inside of a rotating drum until the average particle size of the wollastonite has been reduced below about 5 microns.

4. The process of claim 2 in which the coarse crushed calcium mineral is high grade limestone which has previously been crushed and screened through 20 mesh screens and the hard surfaced bodies are flint pebbles.

No references cited. 

1. A PROCESS FOR GRINDING CALCIUM MINERALS COMPRISING ADDING TO A COARSE CRUSHED MINERAL IN WHICH CALCIUM IS THE PREDOMINANT CATION A SUBSTANCE SELECTED FROM THE GROUP CONSISTING OF OXALIC ACID AND AMMONIUM OXALATE IN AN AMOUNT EQUIVALENT TO AT LEAST ABOUT 0.05 PART BY WEIGHT OF OXALIC ACID PER 100 PARTS OF SAID MINERAL AND SUBJECTING SAID MINERAL WHILE ADMIXED WITH SAID SUBSTANCE TO THE FINE GRINDING ACTION OF A MASS OF TUMBLING AND ROLLING HARD SURFACED BODIES WHICH ARE LARGE IN SIZE RELATIVE TO THE SIZE OF THE INDIVIDUAL PARTICLES OF COARSE CRUSHED MINERAL. 